Electromagnetic flowmeters for measuring the volumetric flow rates of fluids are used extensively in the process control industry. In a magnetic flowmeter, an electromagnetic field is generated having lines of flux which are ideally perpendicular to the longitudinal axis of the flowtube through which the fluid to be metered is conducted. The flux lines are also ideally perpendicular to the transverse axis along which a pair of measuring electrodes are located at diametrically-opposed positions with respect to the flowtube. An electromotive force induced in the fluid by the magnetic field and the moving fluid produces a voltage across the electrodes. That voltage is a function of the process flow rate.
In one type of magnetic flowmeter, the electrodes are electrically insulated from the fluid to be metered, by a dielectric material. Such an "non-wetted" electrode configuration is advantageous in handling troublesome process fluids such as corrosive fluids, sewage and slurries which could damage and corrode the electrodes if in contact therewith.
In flowmeters having non-wetted electrodes, the electrodes are either embedded in the flowtube or disposed on the exterior surface thereof. Either electrode configuration has problems associated therewith. For example, U.S. Pat. No. 4,658,652, Picone et al., discloses a flowmeter having a spool-shaped flowtube formed of a ceramic material and having a pair of electrodes disposed on the exterior surface thereof. The thickness of the ceramic material results in a weak signal at the electrodes, requiring greater amplification and processing of the signal. A reduction in the amount of material separating the electrodes from the process fluids increases the signal strength but decreases the structural integrity of the flowtube and its ability to withstand pressure exerted thereon by the process fluid.
Flowmeters in which the electrodes are embedded within the wall of the flowtube present unique manufacturing problems. For example, Japanese Laid-Open Patent Application No. 58-196419 discloses a method of manufacturing flowtubes characterized by forming an electrode, by a metalizing process, on the outer surface of a first tube of unfired ceramics, and fitting a second tube, also made of unfired ceramics, about the surface of the first tube. Both tubes are then fired together to form a whole unit. A disadvantage of this method is that voids or air pockets form intermediate the two tubes during the firing process. Such voids seriously jeopardize the integrity of the flowtube and its ability to withstand the pressure imparted thereto by the process. More importantly the voids may provide a path for process fluid to penetrate which renders the meter inoperable when the fluid reaches the electrodes.
International Patent Application WO87/01197 discloses a method for manufacturing a ceramic measurement tube in which powdered green ceramic material is placed in a mold with a pair of platinum wire grids located at specific positions. The unfired ceramic material is then compressed and fired. Such a method has several disadvantages. First, the precise placement of the platinum wire grids in a mold of raw ceramic material is very difficult. If the grids or electrodes are not diametrically opposed as precisely as possible, inaccurate flowtube measurements will result. Second, the use of a wire grid results in a relatively small electrode area and a correspondingly weaker signal. Third, platinum is a relatively expensive material and will substantially increase the production cost of the tube.
Accordingly, it is desirable to manufacture a flowtube for use with a magnetic flowmeter which is truly monolithic, with no seams exposed to the process fluid, and in which the electrodes are disposed in close proximity to, but not in contact with, the process fluid flowing therethrough.
A problem associated with both flowmeters having wetted electrodes and those having and non-wetted electrodes involves zero offsets present in the output flow signal of the flowmeter. Such offsets appear as voltages across the electrodes, despite the absence of any fluid flow through the flowmeter which, in an ideal flowmeter, should cause an output flow signal of zero volts. Zero offsets add an error component into the output flow signal from the flowmeter.
When a magnetic field intersects a process fluid flowing through the flow tube at right angles to the magnetic field, an electric field is induced in the process fluid extending between the electrodes. This electric field causes charges to move in the electrodes and their respective leads. Such movement of charges is detected typically by processing circuitry and is desirable and necessary for proper operation of the flowmeter. However, the physical arrangement of the electrodes and leads, in conjunction with the conductive process fluid and the dielectric portion of the flow tube, if any, forms a pick-up loop that is also sensitive to the changing magnetic field. The pick-up loop is essentially a collection of conductors and insulators located inside, outside and within the walls of the flow tube. Any magnetic flux lines which penetrate this pick-up loop cause a voltage to be induced across the electrodes, when the magnetic field changes directions. This induced voltage is manifest as a series of induction spikes present in the signal output of the flow meter, even when the process fluid flow rate is zero.
Previous attempts have been made to insulate the pick-up loop from the lines of magnetic flux and thereby reduce the so called "zero offset voltage" present in the output signal of the flowmeter. In particular, an electromagnetic flowmeter model M2800 commercially available from the Foxboro Company, Foxboro, Mass., 02035, includes a positioning screw for mechanically positioning the plane of the electrode leadouts relative to the magnetic field. By aligning or substantially aligning the plane of the pick-up loop with the magnetic field, the amount of magnetic flux intersecting the pick-up loop is minimized. Accordingly, the amount of zero offset is proportionally minimized.
The alignment of the pick-up loop can be initially adjusted, however, there are installation and process factors which can unbalanced the loop with the magnetic field. In addition, such mechanical devices often add complexity and cost to the flow meter.
In the wetted electrode flowmeters, a similar pick-up loop can form. The geometry of the loop is controlled by the tube design and assembly. Also, the electrical center of the electrodes can move over time, after installation, due to the process coating part of the contact surfaces. Consequently, the alignment of the loop with the magnetic field also changes. With larger flanged flowmeters, eddy currents are created in the mounting flanges and adjacent pipe flanges that cause slight distortions in the magnetic field. These eddy currents and distortions can also change with time as rust forms on the flange bolt holes for example. As a result, the alignment of the pick-up loop with the magnetic field likewise changes.
It is, therefore, an object of the present invention to provide an electromagnetic flowmeter which can accurately measure the flow rate of a fluid passing therethrough.
A further object of the present invention is to provide a monolithic flow tube for use in electromagnetic flowmeter for measuring fluid flow rates.
Another object of the present invention is to provide an electromagnetic flowmeter in which the amount of magnetic flux intersecting the pick-up loop of the flowmeter is adjustable.